From mutualism to parasitism

Mon 19 Dec, 2016

Caribbean coral reefs harbour a diverse wildlife, where sponges stand as a predominant species. In particular, they constitute a habitat for many species with which they are linked through mutualistic relationships.

In exchange for food and shelter, sponges benefit from the protection of their hosts, which also provide “housekeeping services”. However, upon closer inspection, some of these associations look more like a unilateral exploitation…

Researchers investigated the association between the tubular sponge Callyspongia vaginalis and its presumed partner, the brittle star Ophiothrix lineata (a close relative of starfish). The latter, who lives in the tubes of the sponge where it feeds on detritus, was thought to have a cleaning role, thus increasing the efficiency of sponge’s water filtration. Recent experiments have shown, however, that sponges do not benefit from a better growth in the presence of the brittle star, thus gaining no advantage from the association.

The exploitation goes further. Despite the presence of many sponge species, the brittle star presents an almost exclusive fidelity to this single species. Researchers then tried to understand which particularity would initiate such a strong preference. This sponge is characterized by a production of larvae that are released directly inside the tubes. After closer investigation, it appears that the brittle star, peacefully resting in these tubes, actually devours the larvae, eating several of them at a time. Far from being a beneficial partner, the brittle star has seen its status changed into a parasitic species.